Vibration in helicopters caused by unbalance of the main rotor is a problem that has long concerned those in the helicopter field. Excessive rotor-induced vibration can shorten the life of the airframe and installed components such as avionics, and is generally annoying and uncomfortable for the crew and passengers. Helicopter rotor vibration is caused by two primary mechanisms. Inertial unbalance of the rotor is one source of rotor vibration, and results when the center of inertia does not coincide with the rotational axis of the rotor. Inertial unbalance is primarily caused by differences in mass and/or mass distribution between the rotor blades. Accordingly, it is common practice to analyze the inertial balance characteristics of helicopter rotors and to add weights to one or more of the blades so as to inertially balance the rotor. The inertial balancing operation is performed as a maintenance procedure.
Complicating the analysis of inertial unbalance is the fact that rotor vibration can also be caused by aerodynamic unbalance of the rotor, which results when the aerodynamic forces and moments acting on the rotor blades are not the same among all of the blades. This can be due, for example, to differences in blade shape, such as differences in blade twist and/or differences in airfoil shapes, from one blade to another. Such blade shape differences can cause a blade to "mistrack", i.e., to rotate in a plane that deviates from the plane in which other blades are rotating. Ideally, for minimizing aerodynamic unbalance, all blades should rotate in the same plane. Blades that deviate from this plane generate increased rotor vibration and reduced performance. Thus, it is common practice to analyze the tracking of rotor blades and to adjust any mistracking blades in some manner to bring their tracking back into alignment so that all blades track in the same plane.
It will be appreciated that in order to properly correct both inertial and aerodynamic unbalance, it is necessary to determine to what extent a detected vibration is caused by inertial unbalance and to what extent it is caused by aerodynamic unbalance. For this purpose, rotor vibration and tracking analyzers have been developed for use in maintenance procedures to correct rotor unbalance. Typically, rotor vibration and tracking analyzers are maintenance equipment items that are operated when a balancing procedure is to be performed, and are deactivated after the balancing procedure is completed. A rotor vibration and tracking analyzer obtains signals from vibration sensors placed on the rotor and/or airframe, and from a tip path plane sensor that detects to what degree a given rotor blade's tip path plane deviates from a reference plane, which is usually defined as the tip path plane of one blade designated as a "master" blade. The analyzer processes the vibration and tracking data and determines what vibration component is due to inertial unbalance and what component is due to aerodynamic unbalance. The analyzer typically also recommends corrections (e.g., addition of weights to the blade tips) that should be made to one or more identified blades to correct the inertial unbalance, and blade orientation changes that should be made to one or more identified blades to correct the aerodynamic unbalance. Thus, in conventional rotor balancing procedures, maintenance personnel use a rotor vibration and tracking analyzer to analyze the vibration and tracking data while the helicopter is operating, and then the blades are corrected as a ground maintenance operation according to the recommended corrective actions provided by the analyzer. Exemplary systems for analyzing rotor tracking data are shown in U.S. Pat. Nos. 4,531,408; 4,112,774; 4,053,123; and 3,945,256, the disclosures of which are incorporated herein by reference. Additionally, U.S. Pat. No. 3,938,762, incorporated herein by reference, describes a system for discriminating between inertial and aerodynamic unbalance forces using installed electronics with data inputs from a rotor shaft position sensor and airframe vibration sensors, and for recommending mass additions and pitch link adjustments to correct the unbalance.
Inertial unbalance is usually corrected by adding mass to the identified blade(s), as already noted. Aerodynamic unbalance is often corrected by adjusting a pitch link that is attached to the blade and provides rotor cyclic and collective pitch control through cooperation with a swashplate located at the rotor hub. The pitch link is a manually adjustable variable-length link, similar to a turnbuckle. Shortening or lengthening the pitch link by turning it one direction or the other causes the blade's pitch to be increased or decreased so that the blade tracks either higher or lower relative to the airframe. Thus, in helicopters wherein blade tracking is adjusted by adjusting the pitch link length, each mistracking blade is adjusted by turning the pitch link a number of turns from a reference position, and the number of pitch link turns is noted and is often marked on the blade.
In other types of helicopters, aerodynamic unbalance is corrected by making deflections to a trim tab that is attached to the blade, usually at the trailing edge near an outboard position. Rotor track and balance analyzers used with this type of rotor are operable to recommend the amount of angular deflection to be made to the trim tab of each mistracking blade. A maintenance person adjusts the trim tabs according to the analyzer's recommendations.
A significant drawback to the conventional maintenance-based approach to rotor balancing described above is that the rotor must be operated in a number of conditions ranging from ground run, hover, to various forward flight conditions, so that vibration and tracking data can be acquired at each condition. It will be appreciated, however, that a tracking adjustment that might be optimum in hover may very likely not be optimum in forward flight or ground run. Thus, the analyzer determines an across-the-board compromise correction that will reduce unbalance generally, but may not be optimum for some flight conditions. A further drawback of this maintenance-based approach is that it takes a great deal of time to make the initial flights for data acquisition, adjust the blades, and then make validating flights to confirm that the corrective action has produced the desired result. Yet another disadvantage is that the trim tab position that is set can wash out over time, requiring readjustment.
Accordingly, efforts have been devoted toward developing systems capable of making in-flight tracking corrections. For instance, the Kaman SH-2G Super Seasprite helicopter has the ability to perform rotor track adjustments during flight. The Seasprite employs a mechanically controlled aerodynamic servoflap to vary blade pitch, rather than the more-typical direct blade root pitch control through a swashplate and pitch link. The servoflap for each blade is attached to an electromechanical actuator in series with a link in the control system that controls the servoflap position. The electromechanical actuator acts as a variable-length link. Tracking adjustments are made to a given blade by activating the actuator for that blade to increase or decrease its length, thereby changing the pitch of the blade. A drawback to this approach is that because the tracking adjustment actuator is in the primary flight control linkage, it must react the primary flight control loads, and hence must be relatively heavy and robust.
Another helicopter having in-flight tracking adjustment capability is the Bell 214ST, which employs a conventional swashplate and pitch link control system. Similar to the Seasprite, the Bell helicopter also has a variable-length link in the primary pitch control linkage, in the form of an electromechanical device disposed between the pitch horn and pitch link. The electromechanical device is operated to incrementally adjust the pitch of the blade. Thus, this helicopter raises the same concerns as the Seasprite, in that the tracking adjustment device is in the primary flight control linkage.
It would be desirable to provide an in-flight rotor tracking adjustment system that is decoupled from the primary flight control system. One such proposed system is described in U.S. Pat. No. 5,752,672 issued to McKillip, Jr. The McKillip patent discloses a remotely controllable trim tab system employing a plastically deformable trim tab attached to a trailing edge of a rotor blade and integrally formed with an actuator comprising wires of shape memory alloy (SMA) material. Shape memory alloys are one class of so-called "smart" materials that deform when exposed to a stimulus such as electrical current, heat, or magnetism. An SMA material can be plastically deformed at room temperature and, when heated to a higher temperature, the material returns to its original undeformed shape. In the McKillip device, one set of SMA wires are attached between the underside of the rotor blade and the trim tab near its trailing edge, and another set of the wires are attached to the upper side of the blade and the trim tab at locations near the leading and trailing edges of the tab. Electrical current is applied to the upper set of wires to cause them to heat up and shorten so as to plastically bend the trim tab upward, and then the current is discontinued so that the SMA wires cool. The trim tab, since it was plastically deformed, is said to remain in its deformed position. Conversely, when electrical current is supplied to the lower set of wires, the trim tab is bent downwardly. A disadvantage of this system is that it is unlikely to be able to accurately make very small adjustments (e.g., on the order of 0.25 degree) in trim tab angle, since it relies on plastically deforming the trim tab. Accordingly, if the trim tab is bent within its elastic range, the SMA wires in their cooled state may not be stiff enough to hold the trim tab in the deflected position. Furthermore, it would be expected that the SMA wires would respond to temperature changes in the ambient environment and thus the trim tab position would be susceptible to drift as the ambient temperature varied. There is nothing in the McKillip device to positively assure that the trim tab is maintained in a fixed position.
A further example of a system for automatic control of blade tracking is described in U.S. Pat. No. 3,795,375 issued to Lemnios. The system employs a trim tab on each blade, and a small actuator mounted in each blade and connected to the trim tab. The actuator operates in response to the output of a feedback control circuit that includes vibration sensors mounted to the airframe for sensing one-per-revolution pitching moment changes caused by one or more unbalanced blades. The actuator and sensor are said to provide continuous trimming of the rotor blades. The patent does not describe an actuator capable of performing the actuation function. In the Lemnios system, a pulse output is applied to a blade determined to be unbalanced such that the actuator incrementally deflects the trim tab, and the pulse is applied at each rotation of the blade until the unbalance is corrected. The continuous tracking system described in the patent requires the trim tab position loop to be closed between the non-rotating and the rotating frames, through slip rings, since the tracking error solution is a result of incrementally deflecting the tab, analyzing the result in terms of vibration, then applying a further incremental tab adjustment, and so on until a minimum vibration level is reached. Because each blade is adjusted with each rotor rotation, this type of system requires a relatively high operating bandwidth for signal transmissions through the slip rings between the rotating and non-rotating frames. Accordingly, this high bandwidth would limit the types of actuators that could be used; for example, a DC stepping motor combined with a significant reduction drive could be used to provide actuation at such a high bandwidth. Furthermore, any actuator used in the rotor blade must be capable of withstanding the severe operating environment, in terms of very high centrifugal loads (e.g., on the order of 800 g or more) and high vibrations. A DC stepping motor or the like would be too heavy and bulky to use in this environment, and may not be rugged enough for prolonged operation under the severe use conditions. It should also be noted that Lemnios's system does not differentiate between inertial and aerodynamic unbalance.
Another patent showing a remotely controllable blade aerodynamic adjustment system is U.S. Pat. No. 5,224,826 issued to Hall et al. The system is said to be for automatic control of vibrations of the rotor occurring at certain multiples (higher harmonics) of the rotational frequency. The system employs a flap hinged to the blade trailing edge, and a piezoelectric actuator for deflecting the flap up or down. The actuator comprises a beam formed by a pair of plates joined together along their lengths. At least one of the plates is formed of a piezoelectric material, and preferably both plates are piezoelectric. Applying electrical potentials across the plates causes the beam to bend, which deflects the flap. A drawback of the disclosed system is that the electrical potential must be continuously applied in order to hold the flap in the deflected position.
U.S. Pat. No. 5,239,468 describes an automated helicopter maintenance monitoring system, also known as a health usage and monitoring system (HUMS), for collecting airframe and component maintenance data along with rotor balance and tracking data. The system does not perform in-flight corrective action functions, but rather performs diagnostic functions to determine helicopter faults and to anticipate future faults. The system stores vibration data from accelerometers mounted on the airframe adjacent the main rotor, and stores tracking data from a main rotor track sensor.
Other efforts are currently underway to develop maintenance monitoring and prediction systems, such as the program of the Rotorcraft Industry Technology Association (RITA) to develop a HUMS system including a maintenance data recorder (MDR) for acquiring and recording data to aid in evaluating the life of helicopter system components. Current military research programs are also developing a Structural Usage and Monitoring System (SUMS). The difference between HUMS and SUMS is that the HUMS system utilizes signals from existing aircraft instrumentation, such as turbine exhaust gas temperature, transmission chip detection and oil temperature, additional installed instrumentation such as accelerometers, and possible other instrumentation. In contrast, the SUMS system employs a flight regime recognition code that uses flight control positions, engine/rotor "on-off", etc., as inputs to predict component life. However, there is currently no known HUMS or SUMS system or the like that also includes an automated rotor tracking adjustment system having rotor-mounted actuators that can tolerate the severe operating conditions.